The above-mentioned L-type suspension arm is widely used as a lower arm, etc. of an automobile, and a conventional structure is known in which, in order to reduce the weight of the suspension arm, the arm main body is formed from an upper half body and a lower half body, the upper half body being formed by press forming of a plate material so as to have an inverted U-shaped cross-section, and the lower half body being fixed so as to block the lower open end of the upper half body (ref. Patent Documents 1 and 2 below).    Patent Document 1: Japanese Patent Application Laid-open No. 2002-219918    Patent Document 2: Japanese Patent No. 4038890